One frequently used technique in modern drug discovery involves exposing cells bearing a specific drug discovery target to collections of test compounds so that the effect of the various compounds on the cells, through interaction with the expressed specific target, may be evaluated. In typical techniques, the cells are labeled with an indicator material, such as a fluorescent substrate that signals a signal transduction event, allowing the qualitative and/or quantitative nature of the compound and target interaction to be assessed by an instrument which indicates that the target has been contacted by the compound and, more particularly, measures specific properties of the substrate. Information derived from such assays may generally be used to assign relative activity levels to the various compounds being tested. To expand the breadth of information regarding the most active chemical structures, such assays are often performed with large chemical libraries, or with focused but diverse libraries, and employing medium to high throughput methodologies using automated or robotic systems such as liquid handlers and multi-well plate readers.
Flow cytometers, often referred to as fluorescence activated cell sorting (FACS) apparatus, are unique instruments that utilize fluidic systems to align cells in single file; in accordance with conventional flow cytometry technology, the cells are passed at relatively high speeds across intersecting beams of light having specific spectral properties. For example, flow cytometers commonly use coherent laser beams from one or more sources (each having distinct spectral laser lines) to excite specific fluorochromes, such as a signal transduction indicator material, with unique spectral properties on or in the cells. In some circumstances, flow cytometers may offer significant advantages over other analytical instruments such as may be implemented in single cell analyses and spectrally multiplexed measurement applications. On the other hand, flow cytometers have not typically been successfully combined with automated sample mixing and injection mechanisms, particularly systems that allow target-bearing cells to be mixed with test compounds and subsequently injected into the fluidic system of the flow cytometer. In that regard, conventional flow cytometer and other sample analysis techniques are deficient at least to the extent that they are inherently associated with substantial carryover of compound or sample material from one sample to the next, they are generally characterized by relatively low throughput rates, or both.